U.S. patent application number 13/834925 was filed with the patent office on 2014-09-18 for detection of special nuclear material and other contraband by prompt and/or delayed signatures from photofission.
The applicant listed for this patent is Varian Medical Systems, Inc.. Invention is credited to James CLAYTON, Edward J. Seppi.
Application Number | 20140270034 13/834925 |
Document ID | / |
Family ID | 51527012 |
Filed Date | 2014-09-18 |
United States Patent
Application |
20140270034 |
Kind Code |
A1 |
CLAYTON; James ; et
al. |
September 18, 2014 |
Detection of Special Nuclear Material and Other Contraband by
Prompt and/or Delayed Signatures from Photofission
Abstract
In accordance with embodiments of the invention, at least the
potential presence of Special Nuclear Material ("SNM") is
determined by the detection of prompt neutrons, prompt gamma rays,
delayed neutrons, and/or delayed gamma rays from photofission, via
time-of-flight ("TOF") spectroscopic methods. Methods and systems
are disclosed.
Inventors: |
CLAYTON; James; (San Jose,
CA) ; Seppi; Edward J.; (Portola Valley, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Varian Medical Systems, Inc.; |
|
|
US |
|
|
Family ID: |
51527012 |
Appl. No.: |
13/834925 |
Filed: |
March 15, 2013 |
Current U.S.
Class: |
376/154 |
Current CPC
Class: |
G01T 3/005 20130101;
G01V 5/0091 20130101 |
Class at
Publication: |
376/154 |
International
Class: |
G01T 3/00 20060101
G01T003/00 |
Claims
1. A method of examining contents of an object, the method
comprising: scanning an object by first and second consecutive
radiation pulses; checking for the detection of gamma rays during a
first time period starting after the scanning by the first
radiation pulse and ending before a start of scanning by the second
radiation pulse, the first time period being based, at least in
part, on a first expected arrival time of gamma rays at a first
detector resulting from interaction of the first radiation pulse
with Special Nuclear Material within the object, if any; checking
for the detection of neutrons during a second time period starting
after the scanning by the first radiation pulse and ending before a
start of scanning by the second radiation pulse, the second time
period being based, at least in part, on a second expected arrival
time of neutrons at a second detector resulting from interaction of
the first radiation pulse with Special Nuclear Material within the
object, if any; and determining whether the object at least
potentially contains Special Nuclear Material based, at least in
part, on the check for the detection of the gamma rays and the
check for the detection of neutrons.
2. The method of claim 1, wherein the gamma rays are prompt gamma
rays and the neutrons are prompt neutrons, the method comprising:
checking for the detection of prompt gamma rays during the first
time period; checking for the detection of prompt neutrons during
the second time period; and determining whether the object at least
potentially contains Special Nuclear Material based, at least in
part, on the check for the detection of the prompt gamma rays and
the check for the detection of prompt neutrons.
3. The method of claim 2, further comprising: generating the first
and second radiation pulses by: injecting a first radiofrequency
energy pulse into an accelerator; injecting first and second
consecutive charged particle pulses into the accelerator during
injection of the first radiofrequency energy pulse, to accelerate
the first and second charged particle pulses; and impacting target
material by the first and second accelerated charged particle
pulses; wherein the first and second time periods are each between
the injection of the first and second consecutive charged particle
pulses and not during the injection of the first and second
consecutive charged particle pulses.
4. The method of claim 3, wherein: the first and second consecutive
charged particle pulses are separated by from about 100 nanoseconds
to about 300 nanoseconds; and the first and second charged particle
pulses are from about 0.5 nanoseconds to about 10 nanoseconds
long.
5. The method of claim 1, wherein: the first detector is a first
distance from the object; the second detector is a second distance
from the object; a source of the first and second radiation pulses
is a third distance from the object; the first expected arrival
time is based, at least in part, on the first and third distances;
and the second expected arrival time is based, at least in part, on
the second and third distances.
6. The method of claim 1, wherein the gamma rays are delayed gamma
rays and the neutrons are delayed neutrons, the method comprising:
checking for the detection of delayed gamma rays in the first time
period; checking for the detection of delayed neutrons in the
second time period; and determining whether the object contains
Special Nuclear Material based, at least in part, on the check for
the detection of the delayed gamma rays and the check for the
detection of delayed neutrons.
7. The method of claim 6, wherein the first radiation pulse
comprises at least one first radiation pulse and the second
consecutive radiation pulse comprises at least one second radiation
pulse, the method comprising generating the first and second
radiation pulses by: injecting first and second, consecutive
radiofrequency energy pulses into an accelerator; injecting at
least one first charged particle pulse into the accelerator during
injection of the first radiofrequency energy pulse, to accelerate
the at least one first charged particle pulse; and injecting at
least one second charged particle pulse into the accelerator during
injection of the second consecutive radiofrequency energy pulses to
accelerate the at least one second charged particle pulse;
impacting target material by the at least one first and the at
least one second accelerated charged particle pulses to generate
the at least one first radiation pulse and the at least one second
radiation pulse, respectively, for scanning the object; wherein the
first and second time periods start and end between injection of
the first and second consecutive radiofrequency energy pulses and
not during either of the first and second consecutive
radiofrequency pulses.
8. The method of claim 7, comprising: injecting a plurality of
first charged particle pulses into the accelerator during the first
radiofrequency energy pulse, forming a first plurality of
micropulses of radiation, the first plurality of micropulses
forming a first macropulse of radiation; and injecting a plurality
of second charged particle pulses into the accelerator during the
second radiofrequency energy pulse, forming a second plurality of
micropulses of radiation, the second plurality of micropulses
forming a second macropulse of radiation; wherein the first and
second time periods are between the first and second macropulses of
radiation and not during either of the first or second macropulse
of radiation; the method further comprising: checking for the
detection of prompt gamma rays and prompt neutrons between at least
certain of the first plurality of micropulses in the first
macropulse and between at least certain of the second plurality of
micropulses in the second macropulse.
9. The method of claim 1, wherein the radiation pulses comprise
X-ray radiation pulses, the method comprising: scanning the object
by first and second consecutive X-ray radiation pulses.
10. The method of claim 9, wherein the radiation pulses further
comprise neutron pulses, the method further comprising: scanning
the object by third and fourth consecutive neutrons pulses.
checking for the detection of gamma rays during a third time period
starting after the scanning by the third radiation pulse and ending
before a start of scanning by the fourth radiation pulse, the first
time period being based, at least in part, on a third expected
arrival time of gamma rays at a first detector resulting from
interaction of the third radiation pulse with Special Nuclear
Material within the object, if any; checking for the detection of
neutrons during a fourth time period starting after the scanning by
the third radiation pulse and ending before a start of scanning by
the fourth radiation pulse, the fourth time period being based, at
least in part, on a fourth expected arrival time of neutrons at a
second detector resulting from interaction of the first radiation
pulse with Special Nuclear Material within the object, if any; and
determining whether the object at least potentially contains
Special Nuclear Material based, at least in part, on the check for
the detection of the gamma rays and the check for the detection of
neutrons.
11. The method of claim 10, further comprising: determining an
identity of contents of the object based, at least in part, on the
check for the detection of the gamma rays and the check for the
detection of neutrons in the third and fourth time periods.
12. The method of claim 1, further comprising: detecting the first
and second consecutive radiation pulses transmitted through the
object; generating a transmission image based on first and second
radiation pulses; and determining whether the object at least
potentially comprises Special Nuclear Material based, at least in
part, on the spatial image.
13. The method of claim 1, further comprising: determining a
location of the at least potential Special Nuclear Material in the
object based, at least in part, on respective detection times for
the gamma rays and neutrons at the first and second detectors.
14. A method of examining contents of an object, the method
comprising: scanning an object by a plurality of consecutive
macropulses of radiation, the consecutive macropulses comprising
respective pluralities of consecutive micropulses of radiation,
wherein consecutive macropulses are separated by a greater amount
of time than consecutive micropulses; checking for the detection of
prompt gamma rays in respective first time periods between
respective consecutive micropulses of radiation and not during any
of the respective consecutive micropulses, the respective first
time periods being based, at least in part, on respective expected
arrival times of prompt gamma rays at a first detector resulting
from photofission within the object, if any, induced by a
respective consecutive micropulse; checking for the detection of
prompt neutrons in respective second time periods between
respective consecutive micropulses of radiation and not during any
of the respective consecutive micropulses, the respective second
time periods being based, at least in part, on respective expected
arrival times of prompt neutrons at a second detector resulting
from photofission within the object, if any, induced by a
respective consecutive micropulse; checking for the detection of
delayed gamma rays in respective third time periods between
respective consecutive macropulses of radiation and not during
either of the respective consecutive macropulses, the respective
third time periods being based, at least in part, on respective
expected arrival times of delayed gamma rays at the first detector
resulting from photofission within the object, if any, induced by a
respective consecutive macropulse; and checking for the detection
of delayed neutrons in respective fourth time periods between
respective consecutive macropulses of radiation and not during any
of the respective consecutive macropulses, the respective fourth
time periods being based, at least in part, on respective expected
arrival times of delayed neutrons at the second detector resulting
from photofission within the object, if any, induced by the
respective consecutive macropulse; and determining whether the
object contains Special Nuclear Material based, at least in part,
on the checks for the detection of the prompt gamma rays, prompt
neutrons, delayed gamma rays, and/or delayed neutrons.
15. The method of claim 14, comprising: determining that the object
contains Special Nuclear Material if both prompt gamma rays and
prompt neutrons are detected; and confirming that the object
contains Special Nuclear Material if at least one of delayed gamma
rays and delayed neutrons are detected.
16. The method of claim 14, further comprising: generating the
consecutive macropulses of radiation by: injecting consecutive
radiofrequency energy pulses into an accelerator; injecting a
plurality of charged particle pulses into the accelerator during
injection of at least some of the radiofrequency energy pulses to
accelerate the charged particle pulses; and impacting the
accelerated charged particle pulses onto target material to
generate the plurality of micropulses of radiation corresponding to
respective radiofrequency pulses.
17. A system for examining contents of an object, the system
comprising: a support to support an object; a radiation source
configured to generate first and second radiation pulses to scan
the object; at least one first detector positioned to detect gamma
rays resulting from interaction of the radiation beam with the
object; at least one second detector positioned to detect prompt
neutrons resulting from interaction of the radiation beam with the
object; at least one processor configured to: check for the
detection of gamma rays during a first time period starting after
the scanning by the first radiation pulse and ending before a start
of scanning by the second radiation pulse, the first time period
being based, at least in part, on first expected arrival time of
gamma rays at the first detector resulting from interaction of the
first radiation pulse with Special Nuclear Material within the
object, if any; check for the detection of neutrons in second time
periods starting after the scanning by the first radiation pulse
and ending before a start of scanning by the second radiation
pulse, the second time period being based, at least in part, on
second expected arrival time of neutrons at the second detector
resulting from interaction of the respective first of the
respective consecutive radiation pulses with Special Nuclear
Material within the object, if any; and determine whether the
object contains Special Nuclear Material based, at least in part,
on the check for the detection of the gamma rays and the check for
the detection of neutrons.
18. The system of claim 17, wherein: the gamma rays are prompt
gamma rays and the neutrons are prompt neutrons; and the at least
one processor is configured to: determine whether the object
contains Special Nuclear Material based, at least in part, on the
check for the detection of the prompt gamma rays and the check for
the detection of prompt neutrons.
19. The system of claim 17, wherein the radiation source comprises:
an accelerator; a source of radiofrequency energy pulses to inject
a first radiofrequency energy pulse into the accelerator; a charged
particle source configured to inject first and second consecutive
pulses of charged particles into the accelerator for acceleration,
during injection of the first and second radiofrequency energy
pulses; and target material, wherein impact of the accelerated
charged particle pulses on the target cause generation of the first
and second pulses of radiation.
20. The system of claim 17, wherein the at least one processor is
configured to: gate the first detector to a high gain during the
first time period to detect the gamma rays; and gate the second
detector to a high gain during the second time period to detect the
neutrons.
21. The system of claim 18, wherein the at least one processor is
further configured to: check for the detection of delayed gamma
rays at the first detector during third time periods based, at
least in part, on expected arrival times of delayed gamma rays at
the first detector; check for the detection of delayed neutrons at
the second detector during fourth time periods based, at least in
part, on expected arrival times of delayed neutrons at the second
detector; and the at least one processor is configured to:
determine whether the object contains Special Nuclear Material
based, at least in part, on the checks for the detection of the
prompt gamma rays, prompt neutrons, delayed gamma rays, and delayed
neutrons.
22. The system of claim 17, wherein the gamma rays are delayed
gamma rays and the neutrons are delayed neutrons; and the at least
one processor is configured to: check for the detection of delayed
gamma rays at the first detector during third time periods based,
at least in part, on expected arrival times of delayed gamma rays
at the first detector; check for the detection of delayed neutrons
at the second detector during fourth time periods based, at least
in part, on expected arrival times of delayed neutrons at the
second detector; and determine whether the object contains Special
Nuclear Material based, at least in part, on the check for the
detection of the delayed gamma rays and the check for the detection
of delayed neutrons.
23. The system of claim 17, wherein the radiation source comprises:
an accelerator; a source of radiofrequency energy pulses to inject
consecutive radiofrequency energy pulses into the accelerator,
wherein the first and second time periods are between consecutive
radiofrequency energy pulses and not during consecutive
radiofrequency pulses; a charged particle source to inject at least
one pulse of charged particles into the accelerator during
injection of a respective radiofrequency energy pulse, for
acceleration of the at least one charged particle pulse; and target
material, wherein impact of the accelerated charged particle pulses
on the target causes generation of respective pulses of
radiation.
24. The system of claim 17, wherein the first and second detectors
are the same.
25. The system of claim 23, wherein the first and second detectors
each comprise: a plastic scintillator behind the entrance window; a
light reflector surrounding the plastic scintillator; a plurality
of helium tubes behind the plastic scintillator to detect neutrons;
hydrogenous material around the helium tubes; and cadmium around
the detector.
26. The system of claim 17, wherein: the radiation source is
configured to generate first and second pulses of X ray
radiation.
27. The system of claim 26 wherein the radiation source further
configured to generate third and fourth pulses of neutrons to scan
the object; and the processor is further configured to: check for
the detection of gamma rays during a third time period starting
after the scanning by the third radiation pulse and ending before a
start of scanning by the fourth radiation pulse, the first time
period being based, at least in part, on a first expected arrival
time of gamma rays at the first detector resulting from interaction
of the third radiation pulse with Special Nuclear Material within
the object, if any; check for the detection of neutrons in fourth
time period starting after the scanning by the third radiation
pulse and ending before a start of scanning by the fourth radiation
pulse, the second time period being based, at least in part, on a
fourth expected arrival times of neutrons at the second detector
resulting from interaction of the respective first of the
respective consecutive radiation pulses with Special Nuclear
Material within the object, if any; and determine whether the
object contains Special Nuclear Material based, at least in part,
on the check for the detection of the gamma rays and the check for
the detection of neutrons.
28. The system of claim 17, wherein the radiation source is
configured to generate first and second neutron pulses for
scanning.
29. The system of claim 28, wherein the at least one processor is
configured to: determine an identity of the contents of the object
based, at least, in part, on the detected delayed gamma rays and
the detected delayed neutrons.
30. The system of claim 17, further comprising: at least one third
detector to detect X-ray radiation transmitted through the
object.
31. The system of claim 17, wherein: the first detector is a first
distance from the object; the second detector is a second distance
from the object; the radiation source is a third distance from the
object; the first expected arrival time is based, at least in part,
on the first and third distances; and the second expected arrival
time is based, at least in part, on the second and third
distances.
32. The scanning system of claim 17, wherein the radiation source
comprises a source of monoenergetic radiation.
33. The scanning system of claim 32, wherein the source of
monoenergetic radiation comprises an inverse Compton scattering
accelerator.
Description
FIELD OF THE INVENTION
[0001] Radiation scanning of objects and, more particularly,
radiation scanning of objects for Special Nuclear Material and
other contraband based on the detection of prompt and/or delayed
gamma rays and neutrons.
BACKGROUND OF THE INVENTION
[0002] High energy (MeV) X-ray radiation is used to scan cargo
containers and air shipments for contraband, verification of
manifests for customs, and duty collection. The items of concern
may depend on the individual customs agencies for a country, or
individual ports. X-ray radiation may be used to non-intrusively
examine cargo containers and other objects for illegal drugs,
weapons, explosives, chemical agents, and/or biological agents.
Radioactive, fissionable, fissile, and fertile materials, including
Special Nuclear Material ("SNM") that may be used to manufacture
atomic devices, including dirty bombs, may also be smuggled in such
objects.
[0003] Fissile materials, such as uranium-235, uranium-233, and
plutonium-239, may undergo fission by the capture of a photon or
slow (thermal) neutron of sufficient energy. Photon-induced fission
is referred to as photofission. Fission may also be induced by
lower energy photons and neutrons by barrier penetration at a lower
rate than photofission.
[0004] Fissionable materials include fissile materials, and
materials that may undergo fission by capture of fast neutrons,
such as uranium-238. Fertile materials may be converted into
fissile materials by the capture of a slow (thermal) neutron. For
example, uranium-238 may be converted into plutonium-239, and
thorium-232 may be converted into uranium-233. Fissionable,
fissile, and fertile materials are referred to herein as "nuclear
material."
[0005] Special Nuclear Material ("SNMs"), which more readily
undergo fission than other fissile materials, are defined by the
U.S. Nuclear Regulatory Commission to include plutonium,
uranium-233, and uranium enriched in the isotopes of uranium-233 or
-235. Radioactive materials, certain of which may have lower atomic
numbers than nuclear materials (cobalt-60, for example, has an
atomic number of 27), are typically shielded by high atomic number
materials, such as lead (Z=82), tungsten (Z=74), and molybdenum
(Z=42).
[0006] SNM may undergo fission by absorbing a photon having energy
above a fission threshold of the particular SNM. SNM have fission
thresholds of about 5.8 to 6.0 MeV. Photofission releases about 200
MeV of energy in the form of high energy neutrons, gamma rays,
excited fission fragments, and kinetic energy transferred to
fission fragments. The high energy neutrons and gamma rays are
referred to herein as "prompt neutrons" and "prompt gamma rays,"
respectively, because they are released very soon (on the order of
10.sup.-15 to 10.sup.-12 seconds) after fission. On average, 6 to 7
prompt gamma ray photons and 2 to 3 prompt neutrons are generated
in each photofission event, depending on the SNM present. For
example, on average, 2.4 neutrons are emitted per fission of
uranium-235, while on average 2.9 neutrons are released per fission
of plutonium-239. Almost all of the fission fragments are neutron
rich and decay toward a stable valley via beta decay to produce
delayed gamma rays and delayed neutrons, depending on the fission
fragments. These beta decays happen microseconds to hundreds of
milliseconds after emission of the prompt gamma rays and neutrons.
The emission rate of prompt neutrons and prompt gamma rays is about
100 times greater than the emission rate of delayed neutrons. The
emission rate of delayed gamma rays is at least 10 times greater
than the emission rate of delayed neutrons. While gamma rays are
emitted from almost all of the nuclei subject to fission, not every
nucleus will emit a beta delayed neutron. A small amount of fission
may be induced by photons having energies below the fission
threshold, by barrier penetration. The detection of nuclear
material based, at least on part, on the detection of delayed
neutrons, is discussed in U.S. Pat. No. 7,423,273, which is
assigned to the assignee of the present invention and is
incorporated by reference herein.
[0007] Neutron detectors, to detect both prompt and delayed
neutrons, typically comprise three (3) helium-filled tubes
surrounded by a hydrogenous material, such as polyethylene. The
hydrogenous material is covered with a layer of cadmium (Cd), which
has a large capture cross-section for thermal neutrons (2500
barns). The cadmium absorbs background thermal neutrons resulting
from thermalized photoneutron events, preventing their passage to
and detection by the helium tubes. Other neutron detection methods
are known in the art, including scintillators and silicon carbide
(SiC) detectors, for example.
[0008] Gamma rays have been collected by scintillator based
detectors, such as sodium iodide doped with thallium (NaI(Tl)),
barium germanium oxide (BGO), high purity germanium (HPGe),
germanium lithium (GeLi), and plastic scintillators. Inorganic
scintillators have better efficiency than organic scintillators but
may be too expensive for large solid angle arrays required when
examining large objects such as cargo containers.
SUMMARY OF THE INVENTION
[0009] In accordance with embodiments of the invention, at least
the potential presence of Special Nuclear Material ("SNM") is
determined by the detection of prompt neutrons, prompt gamma rays,
delayed neutrons, and/or delayed gamma rays from photofission, via
time-of-flight ("TOF") spectroscopic methods. The use of one or
more TOF spectra enables separate detection of any or all of these
photofission by-products, while decreasing the detection of
background radiation that could interfere with proper
identification of the by-products. In certain embodiments of the
invention, the collection of prompt and/or delayed gamma rays,
and/or prompt and/or delayed neutrons enables identification of the
SNM present, as well as the presence of conventional explosives,
chemical agents, and drugs, based on the unique signatures
(energies) of emissions from respective excited nuclei of
particular elements. The TOF data may also enable localization of
materials in the container. The multiple modes of examination make
it more difficult to shield SNMs, conventional explosives, chemical
agents, and drugs, for example, from detection.
[0010] If it is desired to detect prompt and/or delayed neutrons,
then the radiation pulses need to be separated by sufficient time
to allow the lowest energy neutron of interest to reach and be
detected by a detector, before a new pulse causes generation of new
fission by-products. The X-ray source is pulsed to generate pulses
of X-ray radiation separated by sufficient time to allow the
slowest neutrons resulting from a first pulse to be detected prior
to emission of the next pulse.
[0011] In accordance with one embodiment of the invention, prompt
gamma rays and prompt neutrons are detected. The prompt gamma rays
and prompt neutrons may be detected after scanning by the pulse of
radiation that causes generation of the prompt gamma rays and
neutrons. For the example, the prompt gamma rays and neutrons may
be detected in time periods between consecutive pulses of
radiation. Here, first and second radiation pulses are "consecutive
radiation pulses" if no radiation pulses are provided between the
first and second radiation pulses.
[0012] In accordance with another embodiment of the invention,
delayed gamma rays and delayed neutrons are detected after scanning
by the pulse of radiation that causes generation of the delayed
gamma rays and neutrons, in time periods between consecutive pulses
of radiation. In one example, each pulse of radiation causing
generation of the delayed gamma rays and neutrons is a macropulse
comprising a plurality of micropulses of radiation. In this case,
prompt gamma rays and prompt neutrons may be detected between
consecutive micropulses of radiation during a macropulse, while
delayed gamma rays and delayed neutrons may be detected between
consecutive macropulses. Consecutive macropulses are consecutive
groups of radiation pulses separated by greater time periods than
the time periods between the micropulses within a respective
macropulse. In a compact system, where radiation source and the
object being examined are up to about 20 meters apart, each
macropulse may be from about 3 microseconds to about 10
microseconds long, and each micropulse may be from about 0.5
nanosecond to about 10 nanoseconds long, for example. The time
between micropulses may be from about 100 nanoseconds to about 300
nanoseconds, for example. The exact time periods are determined, in
part, by the size of the scanning unit, including the distances
from the radiation source to the object, the thickness of the
object, and the distance from the object to detectors provided to
detect the respective fission by-products and the expected time of
detection of the various fission by-products.
[0013] In accordance with another embodiment of the invention, the
at least potential presence of SNMs, as well as explosives, drugs,
and certain chemical and biological agents, may be determined by
scanning with both X-ray radiation and neutrons.
BRIEF DESCRIPTION OF THE FIGURES
[0014] FIG. 1 is a schematic diagram of an example of an X-ray
scanning system in accordance with an embodiment of the
invention;
[0015] FIG. 2 is an example of a timing diagram for the operation
of various components of the scanning system of FIG. 1 in
accordance with this embodiment of the invention;
[0016] FIG. 3 is an example of a timing diagram for components of
the X-ray system, to detect delayed gamma rays and delayed
neutrons;
[0017] FIG. 4 is a schematic diagram of a combined gamma
ray/neutron detector for use in the system of FIG. 1; and
[0018] FIG. 5 is a schematic representation of neutron/X-ray source
for use in a second embodiment of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0019] Detectors are gated between a high voltage, detection state
and a low voltage, quiescent state based on a time of flight
("TOF") spectrum of a radiation source and/or expected emissions of
different photofission products, such as prompt gamma rays, delayed
gamma rays, prompt neutrons, and/or delayed neutrons, to eliminate
the interfering photon background. The TOF spectrum takes into
consideration the expected detector arrival times of the
photofission products of interest, based on the emission times and
the speed of the emitted products through air and the container
being examined. The TOF spectrum may take into consideration one or
more of the following: prompt gamma rays (photons) and prompt
neutrons arrive at detectors before delayed gamma rays and delayed
neutrons, photons are faster than prompt neutrons, and neutron
speed may vary based on the kinetic energy from the fission
reaction. Use of the TOF spectrum may also help avoid the detection
of background particles such as cosmic muons and natural radiation
from background sources, while detecting the photofission
by-products of interest.
[0020] FIG. 1 is a schematic diagram of an example of a radiation
scanning system 100 in accordance with one embodiment of the
invention. The radiation scanning system comprises a radiation
source 101. In this embodiment the radiation scanning system 100 is
an X-ray scanning system 100 comprising an X-ray source 101. The
X-ray source 101 comprises an accelerator 102 comprising one or
more accelerating chambers (not shown) driven by an RF source 103,
such as a magnetron or a klystron. A charged particle source 104
injects charged particles into the accelerating chambers of the
linear accelerator 102. The charged particles may be injected as
pulses of charged particles. The charged particles may be electrons
and the charged particle source 104 may be an electron gun, for
example. In another embodiment of the invention, the X-ray source
101 is replaced by a combined X-ray source/neutron source 200, as
discussed further below with respect to FIG. 5. The source 101 may
also be replaced by only a source of neutrons in FIG. 1, as well.
In that case, the source could comprise a cyclotron or DC
accelerator, for example.
[0021] Charged particles injected into the accelerating chambers
while RF power is provided to the accelerating chambers are
accelerated. The accelerated charged particles impact a target 105
to generate X-ray radiation by the Bremsstrahlung effect. A
collimator 107 collimates the generated radiation into a radiation
beam directed toward an object 107 to be inspected, such as a cargo
container. The collimator 107 may collimate the radiation into a
pencil beam, fan beam, or cone beam, for example. The collimator
107 may be an adjustable collimator, such as a multi-leaf
collimator, so that the height and/or width of the beam may be
adjusted, as discussed further below.
[0022] A conveying system 108 supports and moves the cargo
container 108 or other object to be inspected through the system
100. The cargo container 108 may be moved continuously through the
system or in steps, for example. Scanning may take place while the
cargo container is being moved through the system or in between
steps. The cargo container 108 may be rotated while being scanned,
as well. In that case the conveying system would convey the cargo
container to and from a rotatable platform, for example.
[0023] In the example of FIG. 1, a plurality of detectors 109, 110,
112 are provided around the cargo container 107. The first detector
109 is a spatial, integrating detector positioned behind the cargo
container 107 to detect X-ray radiation transmitted through the
cargo container. The first detector 109 may be a photon detector,
such as photodiodes comprising inorganic scintillators, as is known
in the art. Cadmium tungstate (CdWO.sub.4) scintillators may be
used, for example. Amorphous silicon (aSi) detectors, such as
PaxScan.TM. detectors available from Varian Medical Systems, Inc.,
Palo Alto, Calif., may also be used, for example. The spatial
detectors may be in the form of modules assembled into one or more
detector arrays. The first spatial detector 109 is optional.
[0024] The second detectors 110 are neutron detectors to detect
prompt and/or delayed neutrons. The neutron detectors 110 are
provided at least partially around the cargo container 107. Since
neutrons may be emitted in any direction, the neutron detectors 110
may be positioned completely around (4.pi. solid angle) the cargo
container 107. The detectors may be large solid angle detectors,
enabling a large amount of the 4.pi. solid angle around the
container 107 to be covered. The detectors 110 may be in the form
of modules assembled into one or more detector arrays. The
detectors 110 may be positioned the same distance from the center
of the object 107 around a curve, to the extent possible. The
neutron detectors 110 may be supported by a shell-like structure
(not shown), centered around the cargo container 107, as is known
in the art. The shell-like structure may be spherical or nearly
spherical, for example. One or more additional layers (not shown)
of neutron detectors may be provided, if desired, to detect
neutrons not detected by the detector 110. Subsets of detectors
with different resolutions may be provided as well. This may enable
larger solid angle coverage with lower electronics costs.
[0025] Each neutron detector module 110 may comprise cylindrical
proportional counters filled with .sup.3He, for example. The
counters may be surrounded by polyethylene, which may be coated
with a layer of cadmium (Cd), for example, to absorb thermal
neutrons ("slow" neutrons). Fast neutrons are thermalized in the
polyethylene layer before being detected in the .sup.3He detectors.
Only prompt and delayed neutrons are therefore detected in the
.sup.3He detectors. Suitable neutron detectors are commercially
available from Canberra Industries, Meriden, Conn., for example. If
additional layers of neutron detectors are provided, they may have
the same or different configurations than that of the neutron
detectors 110.
[0026] Alternatively, the second, neutron detectors 110 may
comprise semiconducting material, such as organic liquid or plastic
scintillators. The scintillators may comprise sodium iodide (NaI),
thallium (Tl), boron germanium oxide ("BGO"), high purity germanium
(HPGe), or germanium lithide (GeLi), for example. Scintillator
materials that allow pulse shape discrimination, such as gadolinium
(Gd), barium fluoride, or fast/slow scintillators may be used to
further differentiate between the detected photons and neutrons
based on the differing light output caused by the detection of
photons and the detection of neutrons by the scintillator, the
delayed signal as is known in the art. Fast/slow scintillators may
be used to further differentiate between the detected photons and
neutrons based on thermalization of fast neutrons and final
absorption, as is known in the art. Suitable detectors are
available from Saint-Gobain Crystals, Hiram, Ohio, and Eljen
Technology, Sweetwater, Tex., for example.
[0027] A fourth, gamma ray detector 112 is also provided to detect
prompt gamma rays that would be generated by an SNM 114 within the
cargo container 107 from photofission. Prompt gamma rays generated
by the SNM 114 are emitted in all directions. Gamma ray detectors
112 may therefore be provided completely around the cargo container
107, supported by another three-dimensional arcuate or spherical
frame between the radiation source 101 and the cargo container 107.
Due to its proximity to the radiation source 102, the gamma ray
detectors 112 may need to be well-shielded to mitigate the
detection of X-ray radiation emitted by the source 101. Appropriate
shielding techniques are known in the art. The gamma ray detectors
112 may comprise organic, organic liquid, or inorganic
scintillators. The scintillators may comprise sodium iodide (Nal),
thallium (Tl), bismuth germanate oxide ("BGO"), high purity
germanium (HPGe), or germanium plastic scintillator, for example.
An alternative gamma ray detector, that can also detect neutrons,
is discussed below. As above, the gamma ray detector 112 may be a
detector array assembled from modules.
[0028] It may be difficult to differentiate gamma rays detected by
a detector behind the cargo container 107 in the transmission
geometry from X-ray photons transmitted through the container,
because the gamma rays and transmitted photons both arrive at about
the same time and there are typically many more photons from the
X-ray source 101 and the container 107 than gamma rays generated
from photofission. Gamma rays emitted in a direction back towards
the X-ray source 101 are easier to detect and differentiate from
X-ray photons because there are fewer back scattered photons than
transmitted photons. In addition, X-ray photons from the source
that are reflected via Compton scattering will be down shifted in
energy to the range of 250-450 KeV, which can be filtered out by
radiation shielding. The arcuate frame supporting the gamma ray
detectors 112 may therefore be a semi-circular frame (not shown)
between the radiation source 101 and the cargo container 107. The
semi-circular frame may be centered about the cargo container 107.
The radius of the semi-circular frame may be greater than or less
than the radius of the shell-like structure supporting the neutron
detectors. The gamma ray detector 112 and the neutron detectors 110
may be supported by the same frame.
[0029] The detectors 109, 110, 112 are coupled to a processor 116,
such as a microprocessor or computer, through suitable
analog-to-digital processing circuitry. The coupling may be
electrical, via wires, or electromagnetic, via optical fiber. The
detectors 109, 110, 112 may be wirelessly coupled to the processor
116 as well. The computer is coupled to a display 118. The
processor 116 or another processor, which may be a programmable
logic controller, controls operation of the radiation source 101,
as well. A master clock (not shown) is provided to provide timing
for the operations of the various components of the scanning system
100, as described further below.
[0030] During X-ray scanning of a container 107, some of the X-ray
photons will be absorbed by the contents of the cargo container 107
and some will be transmitted and scattered through the container.
The detector 109 detects the transmitted X-ray photons, enabling
determination of attenuation of the X-ray beam and generation of an
X-ray image for analysis. Attenuation is indicative of the
densities of the materials within the cargo container 107 traversed
by the X-ray radiation beam, as a known in the art.
[0031] If an SNM 114 is present in the cargo container 107, some
X-ray photons may be absorbed by the SNM, causing photofission. As
discussed above, photofission results in the emission of prompt
neutrons and prompt gamma rays by the nuclei of the SNM, followed
by delayed neutrons and delayed gamma rays from decay of excited
fission fragments. Subsequent pulses of X-ray photons generated by
the X-ray source 102 will be transmitted through the cargo
container 107, or will be absorbed by the SNM 114, causing a new
chain of photofission emissions.
[0032] Obtaining useful information from the X-ray photons, prompt
neutrons, prompt gamma rays, delayed neutrons, and/or delayed gamma
rays concerning the contents of the cargo container 107 may be
difficult because of the multiple repeating events happening in
overlapping time periods. For example, delayed gamma rays or
delayed neutrons resulting from a first pulse of X-ray radiation
may arrive at the detectors at the same time as or after prompt
gamma rays and prompt neutrons resulting from a subsequent pulse.
Background radiation from the radiation source and other sources
may also be detected, interfering with the test results. For
example, even when well shielded, the detectors 112 may detect
photons emitted from the radiation source 101 directly or reflected
from the cargo container 107 and its contents. Muons from cosmic
rays may also be detected.
[0033] In order for the detectors 110 to detect prompt neutrons
caused by a first photofission event instead of subsequently
generated photons from subsequent photofission events, for example,
in accordance with an embodiment of the invention, time of flight
("TOF") spectra are used to control when the detectors 110, 112 are
gated to a high voltage for detection, based on the expected times
when particular events are expected to occur, and gated to a low
voltage to prevent detection of background signals that could
interfere with the detection of the desired events. In addition, in
accordance with embodiments of the invention, the generation of
radiation pulses may be controlled to allow for the detection of
photofission by-products of interest, between radiation pulses.
[0034] Photons travel at the speed of light through air of
2.998.times.10.sup.8 meters per second (about 0.3 meters per
nanosecond ("ns")). Photons travel through typical inorganic and
organic material in a cargo container 107 at about the same speed.
In one example, if the X-ray source 102 is a distance D1 of 10 feet
(3.3 meters) from the face 107a of the cargo container 107 in FIG.
1, it would take an X-ray photon about 10 ns to reach the face. It
would take an X-ray photon about 5 ns to travel through a cargo
container 107 having a width D2 of 1.5 meters (4.9 feet). If the
distance D3 from the rear face 107b of the cargo container 107 to
the face of the detector 109 is also about 10 feet (3.3 meters), it
would take about 10 ns to travel from the rear face 107b of the
cargo container 107 to the detector 110. It would therefore take
about 25 ns for an X-ray photon to travel from the X-ray source 102
through the cargo container 107 to the detectors 110. As mentioned
above, X-ray photons and prompt gamma rays will arrive at the gamma
ray detectors 112 at about the same time.
[0035] Neutrons resulting from photofission travel at a much slower
speed than X-ray photons, prompt gamma rays, and delayed gamma rays
arising from excited nuclei that decay after the fission event has
occurred. The speed of the neutrons depends on the material being
traversed and the energy of respective resulting neutrons. It would
take about 150 ns to about 250 ns for the slowest prompt neutrons
emitted by SNM within the cargo container 107 to travel the 10 feet
(3.3 meters) from the rear face 107b cargo container 107 to the
detector 110, through air. It would take about 100 ns for prompt
neutrons to travel across a cargo container 107 having a width of
1.5 meters (4.9 feet). If all the detectors 110a and all the
detectors 110b are the same distance D3 from the center of the
cargo container 107, then the travel times of the neutrons to the
detectors will be the same. It would therefore take about 210 ns
for such a neutron to be detected by one of the detectors 110a,
after emission by the X-ray source 102 of the X-ray photon causing
generation of that neutron.
[0036] To facilitate detection of prompt gamma rays and prompt
neutrons without detecting X-ray photons from a current or
subsequent X-ray pulse, in one embodiment of the invention, the
radiation source 101 is configured to generate pulses of radiation
separated by sufficient time for any prompt gamma rays and prompt
neutrons emitted by the SNM 114 to reach the gamma ray detector 112
and the neutron detectors 110a, 110b, respectively, based on TOF
spectra. This is accomplished in one example by suitably
controlling the injection of the electron pulses provided by the
electron gun 104 and suitably controlling the injection of the RF
drive pulses provided by the RF source 103 to the accelerator 102
so that a second electron pulse is not injected until after the
expected arrival time of the prompt gamma rays and/or prompt
neutrons that would result from a radiation pulse generated from
the impact of a first, prior electron pulse.
[0037] During a pulse of RF power, the charged particle source 104,
here an electron gun, fires one or more times to inject one or more
pulses of charged particles, here electrons, into the accelerator
102, thereby generating one or more pulses of X-ray photons during
each RF pulse.
[0038] In one example, in the X-ray scanning system 100 having the
dimensions described above, a series of consecutive X-ray radiation
pulses of from about 0.5 ns to about 10 ns wide, separated by from
about 150 ns to about 250 ns, are generated by the X-ray source
101, during a single pulse of RF power. From 1 to about 50 pulses
of charged particles may be injected into the linear accelerator
102 during an RF pulse, for example. Twenty (20) pulses may be
injected to generate 20 radiation pulses during an RF pulse, for
example. To provide sufficient time between radiation pulses to
detect the prompt gamma rays and prompt neutrons, before the
arrival of X-ray photons from the radiation source 102 in a
subsequent X-ray pulse, in this example, each RF pulse provided by
the RF source 105 is from about 4 microseconds to about 5
microseconds long. If 5 microseconds long, for example, twenty
electron pulses, each pulse being one nanosecond long, may be
injected during the RF pulse, resulting in twenty pulses of X-ray
radiation, each one nanosecond long. In this example, each
nanosecond pulse of X-ray radiation is separated by 250
nanoseconds. Prompt gamma rays and prompt neutrons may be detected
in the periods between consecutive radiation pulses. First and
second charged particles are "consecutive pulses" if no charged
particle pulses are provided between the first and second charged
particle pulses.
[0039] Other time periods for electron injection and RF pulse
injection may be provided. If the distances between the radiation
source 102, the object 107 being scanned, and the detectors 109,
110, 112 are different than that described above, other time
periods may be required, which can be determined by one skilled in
the art based on the teachings herein. In a standoff system, where
the detectors may be 100 meters away from the object being scanned,
for example, consecutive radiation pulses may be separated by from
about 230 nanoseconds to about 350 microseconds, for example.
[0040] If delayed gamma rays and/or delayed neutrons are also to be
detected, the RF pulses may be provided to the accelerator 102 from
about 1 to about 4 milliseconds apart, to enable detection of these
photofission by-products between consecutive RF pulses, as
discussed further, below. If not, each RF pulse need only be far
enough apart to allow system components, such as the pulse forming
network (not shown) in the RF power supply, to recharge.
[0041] In addition, to facilitate the detection of prompt gamma
rays by the gamma ray detector 112 and to decrease detection of
X-ray photons that could interfere with the detection of the gamma
rays, the gamma ray detector 112 is gated to a high gain to detect
gamma rays in between consecutive X-ray radiation pulses. The gamma
ray detection 112 is gated low during and for a time after each
X-ray pulse. The gamma ray detector arrays 112 are not, therefore,
sensitive to the X-ray photons emitted by the X-ray source 101 and
the scattered/leakage X-ray photons. Instead of gating the gamma
ray detector 112 to a high gain in between consecutive X-ray
pulses, the detector may be sampled between pulses by the processor
116, for example.
[0042] FIG. 2 is an example of a timing diagram for operation of
various components of the scanning system 100 in accordance with
this embodiment of the invention. The detection of prompt gamma
rays and prompt neutrons takes place between adjacent radiation
pulses.
[0043] FIG. 2, Line A, shows two consecutive RF pulses 202, 204
injected by the microwave power source 102a into the linear
accelerator 102. The first RF pulse 202 is emitted starting at time
zero (0) and has a length of 5 microseconds. The second 5
microsecond pulse 204 is emitted at 1,000 microseconds in this
example and also has a length of 5 microseconds. Subsequent RF
pulses may be emitted every 1,000 microseconds for 5 microseconds,
or at other times and for different lengths of time.
[0044] FIG. 2, Line B shows the injection of a plurality of pulses
206 of charged particles into the accelerator 101 by the electron
gun 102b, during the time period of the first 5 microsecond RF
pulse 202 of Line A. Twenty (20) consecutive charged particle
pulses 206a-206t may be provided, for example, each having the same
length, during the RF pulse 204 shown in Line A, as well as during
subsequent RF pulses. Each charged particle pulse may have a length
of about 0.001 microseconds and a new pulse may be injected every
0.25 microseconds, for example. Only charged particle pulses 206a
to 206d and 206p to 206t are shown for ease of illustration. In
other examples, different numbers of charged particle pulses may be
provided having different lengths and/or at different rates.
[0045] FIG. 2, Line C shows two radiation pulses 208, 210 emitted
by the X-ray source 101 in response to the first two pulses 206a,
206b of charged particles in Line B. The first radiation pulse 206
is emitted starting at time 0 and lasts for 1 ns (0.001
microseconds). The second radiation pulse is emitted at 250 ns
(0.250 to 16 microseconds) and lasts until 251 ns (0.251
microseconds). If twenty (20) charged particle pulses 206a-206t are
emitted in 5 microseconds, then 20 radiation pulses would be
emitted in the same 5 microseconds period. It is noted that the
nanosecond ("ns") time scale of Line C is shown in Line G of FIG. 2
and is different than the microsecond time scale of Line B. Also,
the microsecond time scales of Lines A and B are different from
each other.
[0046] FIG. 2, Lines D-F show examples of timing diagrams for the
detectors 112, 110 and 109, respectively, with respect to the
nanosecond time scale of FIG. 2, Line G, between the successive
radiation pulses 208, 210. The detector arrays 112, 110, 109 can be
gated to a high (Hi) gain for detection and a low (Lo) gain in a
quiescent state during which no detection can take place.
[0047] FIG. 2, Line D is an example of a timing diagram for the
gamma ray detector 112, between the two radiation pulses of FIG. 2,
Line C. To avoid the detection of X-ray photons from the radiation
source 102 directly, and after reflection by the cargo container,
the gamma ray detector 112 is not gated to a high gain setting
until after an X-ray pulse 208 is emitted and after X-ray photons
from the contents of the cargo container 107 are expected to have
been received. In this example, where the front face 107a of the
cargo container is 10 meters from the radiation source 102, the
cargo container is 5 feet (1.5 meters) wide, and the detector array
112 is 0.5 meters from the rear face 107b of the cargo container,
X-ray photons reflected from the center of the cargo container
would be received by a detector 112 about 6.4 ns after emission of
the X-ray pulse from the X-ray source 101 (time to travel 10 meters
to face of cargo container, 3.3 ns; time to travel from to center
of cargo container back to face 107a, 2.6 ns; time from face 107a
to detector array 110 after exiting cargo container, 0.5 ns). The
detector array 112 is therefore gated to the high gain at a time
after 6.4 ns. In this example, the detector array 112 is gated to
the high gain 10 ns after the start of the X-ray pulse. The
detector 112 may be gated to the high gain at other times after 6.4
nanoseconds, instead.
[0048] The gamma ray detector 112 is kept at the high gain to
detect gamma rays until just prior to the emission of the next
X-ray pulse, such as until about 5 ns before the second X-ray pulse
210 is generated at 250 ns (245 ns after emission of the first
pulse). It is then gated to the low gain to be in the low,
quiescent state prior to the start of the next X-ray pulse 210 to
avoid detection of stray photons from the radiation source 102
after emission of the radiation pulse. It may be gated to the low
gain 5 to 10 ns prior to the next X-ray pulse, for example.
[0049] FIG. 2, Line E shows operation of the neutron detectors 110,
between the first two radiation pulses 208, 210 shown in FIG. 2,
Line C. If the peak radiation energy is 20 MeV, and the distance
from the center of the cargo container 107 is 3.0 meters, for
example, prompt neutrons are expected to arrive at the neutron
detectors 110 starting at about 40 nanoseconds. The detectors 110
are therefore gated to the high voltage prior to that time, at 25
ns, for example, to ensure detection of the prompt neutrons. They
are also turned off before the next X-ray pulse, such as at 245 ns,
in this example.
[0050] FIG. 2, Line F is an example of a timing diagram of the
operation of the detector 109, which detects photons transmitted
through the cargo container 107. The spatial detector 109 is always
gated to the high gain to detect transmitted photons. It is not
necessary to gate the spatial detector 109 to a low gain but that
is an option.
[0051] The detectors 110, 112 are similarly gated high and gated
low with respect to the remaining pulses 206c-206t and the
corresponding radiation pulses during the 5 microsecond RF pulse
202, and the same pattern may be repeated during the second RF
pulse 204 and subsequent RF pulses.
[0052] The detection of prompt gamma rays by the detector array 112
or prompt neutrons by the detector arrays 110a, 110b is suggestive
of the presence of SNM. The coincident detection of both prompt
gamma rays and prompt neutrons confirms the presence of SNM, when
the radiation energy has a peak value in the range of from about 6
MeV to about 18 MeV. To further confirm that an SNM is present, the
presence of delayed gamma rays and/or delayed neutrons may be
detected.
[0053] Delayed gamma rays are emitted from about 25 microseconds to
about 1,000 microseconds after absorbing an X-ray photon. Delayed
neutrons are emitted from about 10 microseconds to about 1,000
microseconds after absorbing an X-ray photon. RF pulses and charged
particle injection may also be separated by sufficient time to
allow for detection of delayed gamma rays and delayed neutrons, in
accordance with another embodiment of the invention. The radiation
generated during the RF pulse 202, for example, comprises the
plurality of radiation pulses corresponding to the charged particle
pulses 206a-206t in FIG. 2, lines B, and is referred to as a
"macropulse." Similarly, the plurality of charged particle pulses
injected during each RF pulse is referred to as a charged particle
or electron "macropulse," or "macropulse."
[0054] FIG. 3 is an example of a timing diagram for components of
the X-ray system 100, to detect delayed gamma rays and delayed
neutrons. The timeline of FIG. 3, Line G, which shows a microsecond
scale, applies to all of the timelines A-F. Detection of delayed
gamma rays and delayed neutrons takes place between the RF pulses
and resulting plurality of corresponding X-ray macropulses. The
plurality of X-ray pulses resulting from each RF pulse is referred
to as a radiation macropulse or macropulse.
[0055] FIG. 3, Line A shows the first and second RF pulses 202, 204
of FIG. 2, Line A, emitted starting at times 0 and at 1000
microseconds and provided to the accelerator 102. FIG. 3, Line B,
shows two charged particle macropulses 302, 304 injected into the
accelerator 102 at the same times as the RF pulses 202, 204. FIG.
3, Line C shows two radiation macropulses 208, 210 resulting from
the RF pulses 202, 204 and charged particles macropulses 302, 304
of FIG. 3, Lines A and B, starting at times 0 and 1,000
microseconds. In this example, each charged particle macropulse and
each radiation macropulse comprises twenty (20) charged particle
pulses and twenty (20) radiation pulses, respectively. In other
examples, each macropulse may comprise a different number of
charged particle and radiation pulses.
[0056] FIG. 3, Lines D, E, and F are examples of timelines for the
operation of the gamma ray detector arrays 112, the neutron
detector arrays 110, and the spatial detector array 108,
respectively. As mentioned above, the spatial detector array 108
may be on all the time, as shown in Line F.
[0057] FIG. 3, Lines D and E show the detector arrays 112, 110
gated high during a portion of the radiation macropulses 208, 210.
The multiple, closely spaced lines indicate the multiple on/off
cycles of the detectors in this time period, a portion of which is
shown in FIG. 2. In this embodiment both detector arrays 112, 110
are also gated high between the radiation macropulses 208, 210, to
detect delayed gamma rays and delayed neutrons, respectively. In
this example, both detector arrays 112, 110 are gated high starting
at 100 microseconds, but other start times may be selected between
the X-ray radiation macropulses pulses 208, 210. The detectors 110,
112 are switched to the low gain just prior to the start of the
second X-ray/radiation macropulse 210, such as at 900 microseconds,
for example.
[0058] If delayed gamma rays and delayed neutrons are not to be
detected, the RF pulses may be closer together. In this case, the
RF pulses need only be separated by enough time for the system 100
to recharge. If the radiation source 101 is a DC accelerator, the
nanosecond pulses may be provided continuously.
[0059] The accelerator of the X-ray source 101 may be a linear
accelerator 102. An S-band, 3 GHz linear accelerator, for example,
such as an M9, K9, or K15 linear accelerator available from Varian
Medical Systems, Inc., Palo Alto, Calif., may be used, for example.
The Varian linear accelerators output at least 360 pulses per
second. They may be configured to generate the radiation pulses at
a frequency of from about 200 to 1,000 times per second (Hertz), by
suitable selection and control of the RF source 103 and the
electron gun 104.
[0060] Other types of accelerators may be used, as well. For
example, the accelerator may be a DC accelerator, as mentioned
above.
[0061] A monochromatic X-ray source employing inverse Compton
scattering of a laser beam from a high energy (MeV) accelerator may
also be used. Use of a monoenergetic beam may provide certain
advantages because since all the photons have the same selected
energy and can be tuned to optimize results. In one example, the
beam energy is adjusted to be near the photofission thresholds of
SNMs of 5.8-6.1 MeV. In another example, the beam energy could be
higher than the photofission threshold to overlap the maximum in
the photofission cross-sections at 14-15 MeV. If potential SNM is
identified at the photofission threshold, then scanning can be
repeated a the maximum photofission cross-sections. If SNM is
indicated at both energy regions, then it is very likely that SNM
is present. A further indication of the presence of SNM would be if
there is no indication of SNM at energies below the photofission
threshold, such as at 5.5 MeV, for example. It may be easier to
adjust these energies with a monoenergetic beam than it would be
with a Bremsstrahlung X-ray source. Monochromatic X-ray sources
employing inverse Compton scattering of a laser beam from a high
energy (MeV) accelerator are known in the art. (See, for example,
Holder, D. J., et al., "COBALD-An Inverse Compton Back-Scattering
Source at Darsebury," Proceedings of EPAC08, Genoa, Italy, MOPCO40,
pages 160-162; Shimada, M., et al., "Inverse Compton Scattering of
Coherent Synchrotron Radiation in an Energy Recovery LINAC,"
Physical Review Special Topics-Accelerators and Beams, 13, 100701
(2010). As used herein, the term "monochromatic" means that the
value of the full width at half maximum of the beam divided by the
nominal energy of the beam is no greater than 1%. Use of a
monochromatic radiation source is not required.
[0062] A laser plasma accelerator may also be used as an inverse
Compton scattering source to provide high energy electrons. Laser
plasma accelerators have been used to create electron beams in
excess of 1 GeV of kinetic energy, as described, for example, in
Leemans, et al., "Laser guiding for GeV laser-plasma accelerators,"
Phil. Trans. R. Soc. A (2006) 364, pp. 585-600; and Leemans, et
al., "GeV electron beam from a cm-scale accelerator," Lawrence
Berkeley National Laboratory, May, 4, 2005, for example, which are
incorporated by reference herein. Lower energy laser plasma
accelerators are described in Malka, et al., "Medical Applications
with Electron Beam Generated by Laser Plasma Accelerators," Proc.
of SPIE Vol. 6881, 688110B-1-688110B-2 (2008); and Malka, et al.,
"Principles and applications of compact laser-plasma accelerators,"
nature physics Vol. 4, June 2008, pp. 447-453. To create photons
that are in a range relevant to the detection of SNM in accordance
with embodiments of the invention, electrons having kinetic
energies in the range of from about 300 MeV to about 650 MeV may be
made incident on a laser pulse of from about 1 to about 10 joules,
for example. The photons in the laser pulse are upshifted in energy
to be in the range of from about 5 MeV to about 16 MeV. The exact
energy of the quasi-monochromatic beams are determined by the
incident beam parameter for the electrons and the laser light.
[0063] Laser plasma accelerators are readily tunable to change the
beam energy, enabling the full range of energies from SNM
thresholds to maximum photofission peaks to be examined, such as
from about 5 MeV to about 16 MeV, for example. The laser plasma
accelerator system also has short bursts with beam pulses of from
10 femtoseconds to 50 femtoseconds.
[0064] In order to generate the plurality of nanosecond radiation
pulses described above, the electron gun 104 is configured to
inject very short, high voltage pulses of 10 nanoseconds or less in
pulse duration, for example, and a few hundred or thousand volts.
In the example of FIG. 2, the pulses are 0.25 microseconds apart.
They may be from 0.10 to 0.30 microseconds apart, for example.
[0065] The electron gun 104 may be a gated electron gun, controlled
by a modulated RF field on the grid or anode by modulating the grid
voltage or the anode voltage, for example. A coaxial line-type
pulser comprising high current hydrogen thyratrons and an auxiliary
L-R-C circuit to increase pulse repetition frequency may be used,
for example, as described in Koichi, et al. "Generation of High
Intensity Beams of Nanosecond Pulse Duration by Electron LINACS,"
Japanese Journal of Applied Physics, Vol. 9, No. 11, November 1970,
which is incorporated by reference herein. The use of a hard tube
pulser comprising high-vacuum tubes, and a coaxial pulse-sharpening
atmospheric gap, are also described. The pulser could also comprise
MOSFETS or IGBTS, as in known in the art.
[0066] Grid modulation techniques currently used in electron guns
driving inductive output tubes ("IOT"), where the voltage on the
grid is modulated at a high rate, may also be appropriate to drive
the electron gun 104 of the radiation source, as described in
Zolfghari, et al. "Comparison of Klystron and Inductive Output
Tubes (IOT) Vacuum-Electrode Devices for RF Amplifier Service in
Free-Electron Laser," Proceedings of EPAC 2004, Lucerne,
Switzerland, pp. 1093-1095; and Orrett, et al., "IOT Testing at the
ERCP," Proceedings of EPAC 2006, Edinburgh, Scotland, pp.
1382-1384, which are incorporated by reference herein.
[0067] In another alternative, a laser pulse in a laser driven
photocathode may be modulated to provide an electron gun train, as
is known in the art. Another option is to chop a beam of electrons
generated by a direct current (DC) gun, at a desired rate, as is
also known in the art.
[0068] If a laser plasma accelerator is used, as described above,
an electron gun is not needed because the plasma is the source of
the electrons.
[0069] The target material 105 of the accelerator 102 may comprise
tungsten, tantalum, beryllium, or other target materials known in
the art. The target material 105 may also comprise deuterium in the
form of lithium deuteride, for example.
[0070] The gamma ray detector 112 may also be configured to detect
neutrons. FIG. 4 is a schematic diagram of a combined gamma
ray/neutron detector 400 for use in the system 10 of FIG. 1, in
accordance with another embodiment of the invention, to detect
gamma rays and neutrons, while blocking low energy neutrons, such
as thermal neutrons, from detection. The detector 400 may be used
as the detector 110 or in addition to the detector 110. The
detector 400 applies the Cerenkov Effect, where radiation is
emitted when a charged particle passes through a medium at a
velocity exceeding the speed of light in that medium. The velocity
of the particle (.beta.) relative to the speed of light is
.beta.=1/n cos(.theta..sub.c), where "n" is the index of
refraction, which sets a limit on the detection capability of the
detector. For most plastic scintillator materials, .beta. for
electrons and positrons is greater than a few hundred KeV.
[0071] The combined detector 400 in accordance with this embodiment
comprises a layer of a plastic scintillator 402, to detect the
gamma rays. The plastic scintillator 402 may comprise polyvinyl
toluene (C.sub.10H.sub.11) and organic fluors, such as BC 420,
available from Saint-Gobain Crystals, Hiram, Ohio, for example.
According to a specification provided by Saint-Gobain, BC 420 is
said to have low self-absorption, a time constant of 1.5 ns, a
light output (% Anthracene, where Anthracene light output is 40-50%
of Nal(TL)) of 64, a wavelength of maximum emission of 391
nanometers, a decay constant (main component) of 1.5 ns, a bulk
light attenuation length of 110 cm, a refractive index of 1.58, an
H-C ratio of 1.102, a density of 1.032, and a softening point of
70.degree. C. Saint-Gobain BC 408, BC 501 or BC 720 plastic
scintillators could also be used, for example.
[0072] A light reflector 404 is provided around the plastic
scintillator 402. The light reflector 402 collects light emitted
from the scintillator and transports the collected light to
photomultiplier tubes (not shown), as is known in the art. The
light reflector may comprise poly (methyl methacrylate) ("PMMA").
The PMMA may include a wave shifter dopant, as is known in the
art.
[0073] A thin metal converter 406 is in front of the plastic
scintillator. A plurality of Helium-3 tubes 407 are positioned
behind the plastic scintillator 402, to detect neutrons. In this
example, four Helium-3 tubes are provided. More or fewer Helium-3
tubes 407 may be provided. The Helium-3 tubes 406 are surrounded by
a hydrogenous material 408, such as polyethylene, to slow the fast
neutrons so that they can be captured by the helium-3 tubes 407.
The entire assembly is contained within cadmium shielding 410,
which absorbs thermal reactors.
[0074] Neutrons are detected by the Helium-3 tubes 406, while
photons are detected by the plastic scintillator 402, which also
acts as a moderator to slow neutrons. This facilitates detection by
the helium tubes 406.
[0075] The metal converter 406 converts photons with energies of
greater than 1.02 MeV into e.sup.+e.sup.- pairs that cross the
plastic scintillator 402 and emit Cerenkov radiation. The Cerenkov
radiation is detected by photomultiplier tubes or photodiodes (not
shown). It also further reduces the passage of low energy
background radiation. The converter 406 may comprise lead,
tungsten, or tantalum, for example.
[0076] Most of the X-ray photons reflected from the cargo container
107 by X-ray radiation from the X-ray source 101 have energies
below the Cerenkov threshold, due to absorption and large angle
Compton scattering. Such X-ray photons will not, therefore, be
detected. This reduces the detection of photons that are not of
interest, facilitating the detection of prompt gamma rays from
prompt fission events.
Analysis
[0077] The coincident detection of prompt gamma rays and prompt
neutrons is indicative of the possible presence of SNM. The
detection of delayed gamma rays and/or delayed neutrons provides
further confirmation of the presence of SNM. As noted above, on
average 6 to 7 prompt gamma ray photons and 2 to 3 prompt neutrons
are emitted per photofission event, depending on the SNM. The
detection of two or more prompt gamma rays and 6 to 7 prompt
neutrons from the same region within the object, in between
consecutive radiation micropulses, would therefore be highly
indicative of the presence of SNM in that region. It is believed
that SNM as small as about 100 cubic centimeters, which is about 19
kilograms of highly enriched uranium, or smaller, may be detected.
If only delayed gamma rays and/or delayed neutrons are detected, or
if the system is configured to only detect delayed gamma rays or
delayed neutrons, then the presence of SNM is possible and further
examination is required to increase the probability of
detection.
[0078] The approximate location of a possible SNM may be determined
in a variety of manners. For example, if the radiation beam is a
fan beam or a cone beam, the emission of photofission by-products
can be narrowed to within one or a few scanned slices or regions.
The location can be further narrowed by reducing the height and/or
width of the scanning core beam or fan beam, by the collimator 117.
If the radiation beam is a pencil beam, the location of the
possible SNM can be determined to be within smaller regions.
[0079] The approximate position of an SNM without a scanned region
may also be further refined within the volume of the slice or
region by TOF spectra, taking into consideration the detector
resolution. The inherent time structure in the arrival times of
prompt and/or delayed gamma rays, and/or prompt and/or delayed
neutrons may enable the localization of the position of the SNM, by
determining the depth of the SNM in the object, for example. The
location in the other two dimensions may be determined from the
spatial resolution of the detectors, as is known in the art.
Localization would be advantageous in the inspection of cargo
containers or other large objects since it facilitates the
performance of additional tests or examinations of the suspect
region or object, speeding the identification process.
[0080] For example, if the SNM 114 is at or near the center of the
cargo container 107, and all the detectors 110 are at about the
same distance from the center of the cargo container, then all the
detectors will detect the prompt neutrons at about the same time.
If, however, the SNM 114 is at a position within the cargo
container 107 that is closer to one or the other detectors 110,
that detector will detect the prompt and/or delayed neutrons and/or
gamma rays before the others, enabling the approximate location of
the SNM within the cargo container to be pinpointed. More detailed
analysis of the detection times by the different detector arrays
110, including the gamma detector array 112, enables better
determination of the location of the SNM. The approximate location
of an SNM may also be determined based radiographic imaging, as
well.
[0081] In another example, in a primary scan, an image is generated
based on the spatial detector 109. The image may be a dual energy
image of the cargo container 107 and a fused image may generated
based on images at each energy, for example. If suspect regions are
identified on the image or fused image, subsequent secondary scans
of the suspect area may be performed with X-ray beams of at least 9
MeV, for example. If the suspect regions comprise SNMs,
photofission should be induced. When the X-ray beam is off, delayed
neutron and delayed gamma signatures are detected, as discussed
above. Since the size of the container 107 is known, the location
of the walls can be determined from the first scan, the location of
the SNM 114 may be determined. A maximum likelihood algorithm may
be used to determine the localized three-dimensional object
emitting the signatures, based on a comparison of the detection
times and the decay groups, by the processor 116, for example.
Since the expected decay groups of SNMs for delayed gamma rays and
delayed neutrons are known, the identity of the SNM 114 may also be
determined, based on the energy, time, and pulse shape of the
detected neutrons and photons, as is known in the art. If prompt
gamma rays and/or prompt neutrons are also detected, the data may
be used to further confirm the type of material present.
[0082] It is noted that there will be some "smearing" of data due
to the fact that not all delayed neutrons and delayed gamma rays
are emitted at the same time. The sample also has a finite size.
The precision of the location identification in the cargo
conveyance may therefore be on the order of a few centimeters.
[0083] Further confirmation of the presence of SNM may be provided
in dual energy systems by scanning a suspect region (a region where
photofission by-products may have been detected) with radiation
energy above the photofission threshold and radiation below the
photofission threshold. The detection of photofission by-products
at the energies above the threshold but not below the threshold
would be highly indicative of the presence of SNM in the
region.
[0084] If the radiation source 101 is a source of monochromatic
radiation, as discussed above, then energies of slightly above and
below the photofission threshold of 5 MeV to 7 MeV of a particular
SNM may be used. Uranium-235, for example, has a photofission
threshold between 5.7 and 5.8 MeV. Energies from about 250 KeV to
500 KeV or more above and below the threshold may be used, for
example, such as high energy of from about 6 MeV to about 7 MeV,
and a low energy of from about 4 MeV to about 5 MeV,
[0085] for example. If the radiation source 101 generates
Bremsstrahlung radiation, which is not monochromatic, then the
scanning energies may be 500 KeV or more above and below the
photofission threshold. The detected energy radiation source 101
maybe rapidly cycled between the high and low energies.
[0086] Another technique to increase the confidence that SNM is
present after the detection of photofission by-products is to
review spatial images of the suspect region based on radiation
transmitted through the object and detected by the spatial detector
110. If a region of high density is found on the image, the
likelihood that SNM is present is also increased. In addition, if
no legitimate high density material is identified on the manifest,
the presence of the high density region and photofission
by-products is highly indicative of the presence of illegitimate
SNM.
X-Ray/Neutron Scanning
[0087] In accordance with a second embodiment of the invention, the
at least potential presence of explosives, drugs, and certain
chemical and biological agents, as well as SNMs, may be determined.
In this embodiment, neutron scanning is used along with X-ray
scanning to determine the elemental content of the cargo container
107 or other such object. Since neutrons have a cross-section 7 to
8 times greater than photons, they are more likely to cause fission
in an SNM 114 in the cargo container 107.
[0088] FIG. 5 is a schematic representation of an example of a
neutron/X-ray source 200 for use in this embodiment of the
invention, comprising a linear accelerator 202. The linear
accelerator 202 comprises an accelerating chamber 204 comprising a
plurality of acceleration chambers (not shown), an electron gun
206, and an RF power supply 208, as in the radiation source 102. A
headend 210 is shown including a drift tube 212 at the output of
the acceleration chamber 202, to receive accelerated charged
particles, such as electrons, exiting the accelerator. A first
target 214 comprising tungsten, for example, is positioned within
the drift tube 204. Impact of accelerated charged particle on the
tungsten target 214 causes generation of Bremsstrahlung X-ray
radiation, as is known in the art. The neutron/X-ray source 200 may
replace the X-ray source 101 in FIG. 1, in the second embodiment.
The linear accelerator 202 may be configured to accelerate
electrons to two or more energies in an interlaced manner, as
described in U.S. Pat. Nos. 8,198,587 and 8,183,801, for example,
which are assigned to the assignee of the present invention and are
incorporated by reference herein. Multiple linear accelerators or
X-ray sources may be used to generate the multiple X-ray energies,
as well. The use of multiple linear accelerators is also discussed
in U.S. Pat. No. 8,198,587.
[0089] In accordance with this embodiment, a second, neutron
generating target 216 is provided in the headend 210, selectively
positionable behind the first, tungsten target 214. The second
target 216 may comprise beryllium (Be) or deuterium (D) for
example, which generates neutrons from the impact of X-ray photons
generated by the first target 216. Neutrons are generated by a
gamma/neutron (.gamma., n) process. The second target 216 may be
selectively moved behind the first target 214 by a mechanism,
schematically represented by the arrow 218, such as a plunger or a
rotating wheel (not shown) driven pneumatically by an air cylinder,
for example. The plunger or rotating wheel may be driven
electromagnetically, as well.
[0090] Alternatively, two separate targets may be provided in
different locations. A first, X-ray generating target may comprise
tungsten, for example, to generate X-ray radiation and the second,
neutron generating target may comprise tungsten and beryllium, for
example, to generate neutrons. The path of the accelerated electron
beam may be directed toward one or the other target by a magnetic
field, for example. Neutrons may also be induced by bombardment of
lithium (p, n).sup.7 Li or boron (d, n) .sup.11B by protons or
deuterons accelerated by a cyclotron or radio frequency quadrapole
(RFQ) accelerator, for example, as is also known in the art.
[0091] In accordance with this embodiment, X-ray scanning is
performed with the second target 216 in its first position away
from the first target 214. The X-ray scanning with the first target
214 may be performed as described above with respect to the first
embodiment to detect prompt and/or delayed gamma rays and neutrons,
which are indicative of the presence of SNMs, and/or conventional
X-ray scanning may be performed.
[0092] After sufficient data is collected, the second target 216 is
moved into its second position behind the first target 214. A
suitable time of flight spectrum to control the detectors as in the
examples above may be developed, with longer time periods to detect
photofission by-products resulting from the impact of neutrons on
SNM or other materials, based on the distances between the
radiation source 200, the cargo container 107, and the detectors
109, 110, 112. Some X-ray radiation may pass through the second
target 216 without causing generation of neutrons, and scan the
container 107. The TOF spectra may be adapted to ignore the results
of the interaction of the X-ray radiation with the cargo container
107, during neutron scanning, or the X-ray data may also be used
through detection by the spatial detector 109 to generate one or
more images.
[0093] If the system is capable of dual or multi-energy X-ray
scanning, such scanning may be performed to determine whether high
atomic number material is at least potentially present, as
described in U.S. Pat. Nos. 8,290,120, 8,263,938, 7,636,417,
7,423,273, and 7,257,188, which are assigned to the assignee of the
present invention and are incorporated by reference herein. High
atomic number material may be defined as material having an atomic
number greater than a predetermined atomic number, as described in
these patents.
[0094] Then neutron scanning may be performed at two or more
radiation energies as well, after moving the second target 216 into
its second position behind the first target 214. The neutrons
resulting from the impact of the second target 216 will have
different energy distributions due to the dual energy scanning. The
neutron energy may range from the KeV to MeV range since low energy
neutrons will have a lower velocity.
[0095] For example, when scanning with 9 MeV Bremsstrahlung
radiation, neutrons will be generated having energies of from a few
hundred keV to 6.9 MeV. When scanning with 6 MeV Bremsstrahlung
radiation, the energy endpoint will be tens of keV to 3.9 MeV. To
detect the neutrons across the range of energies, the electron gun
may be gated off longer than described above, to provide more time
to detect the neutrons. Low energy neutrons without enough energy
to penetrate through the cargo container 107 may be filtered by a
block of polyethylene, for example, between the radiation source
200 and the cargo container 107.
[0096] The X-ray image can provide information on elemental ranges
of the components of the cargo container 107, while the neutron
data can further clarify the elemental information through various
mechanisms. The delayed neutrons and delayed gamma rays may be
emitted by different isotopes of an SNM, from different induced
fission events, enabling identification of the material. For
example, neutron scanning can yield neutron elastic scattering
data, which is indicative of the density and types of materials
present. Neutron scanning can also cause inelastic scattering,
resulting in excited nuclei. The excited nuclei decay, emitting a
prompt gamma ray having a unique energy indicative of the emitting
nuclei. Detection of the energy of the photons via the first
detector 109, for example, can therefore be indicative of the
contents of the cargo container. If carbon, nitrogen, and oxygen
are found to be present, the ratio of these elements may be used to
determine if explosive material is present, as is known in the art.
U.S. Pat. No. 5,098,640 describes techniques for determining
whether contraband is present based on three-dimensional images of
hydrogen, carbon, nitrogen and oxygen within an object derived from
gamma ray detection resulting from fast neutron scanning, for
example.
[0097] The presence of chlorine, phosphorus, arsenic, and sulphur
can also be determined based on detected gamma rays, as is also
known in the art. Illegal drugs, such as cocaine and morphine,
which are often transported in chlorinated forms, may be detected
based on a gamma ray signature indicative of the presence of
chlorine, for example, as is known in the art. The photons can be
detected by an array of photon counting detectors, for example,
such as the gamma ray detectors 112.
[0098] Since neutrons, both fast and slow, of sufficient energy can
also induce fission in SNMs resulting in fission by-products,
scanning by the X-ray/neutron source 200 can be used to detect the
presence of SNMs by neutron scanning, as well as by X-ray scanning.
Prompt and/or delayed gamma rays and neutrons may be detected based
on TOF spectra, as discussed above with respect to X-ray scanning.
The expected detection times may be determined in a similar manner
as that discussed above. For example, prompt neutrons, detected
based on TOF spectra, in a similar manner as described above,
generated by the neutron probe beam in an SNM would cause an
enhancement in the energy region above 3 MeV to 4 MeV, indicating
that an SNM is at least potentially present and is creating
neutrons at this energy. Localization of SNM and other contraband
may also be provided, as described above. Neutron energies of above
6 MeV may be used, for example.
[0099] The ability to induce fission by scanning by both X-ray
radiation and neutrons makes it difficult to avoid detection of
SNMs by shielding. The presence of shielding that can prevent
transmission of either or both of X-rays and neutrons would itself
be identified as a suspect, high density region based on
transmission images or data collected by the spatial detector 109
and analyzed in accordance with the teaching of U.S. Pat. Nos.
8,290,120, 8,263,938, 7,636,417, and 7,257,188, for example, which
are assigned to the assignee of the present invention and are
incorporated by reference herein. Such an identification may prompt
further examination in accordance with embodiments of the
invention.
[0100] While discussed above with respect to moving an object
through a radiation beam generated by the radiation source,
embodiments of the application are equally applicable to a system
in which the radiation source is moved horizontally or vertically
with respect to a stationary object. The object may also be moved
vertically and/or rotated with respect to a stationary or movable
source.
[0101] One of ordinary skill in the art will recognize that other
changes may be made to the embodiments described herein without
departing from the spirit and scope of the invention, which is
defined by the following claims.
* * * * *